Trunked communication systems are known to comprise a plurality of communication units, a limited number of communication resources that are transceived Via a predetermined number of repeaters, or base stations, and a communication resource allocator that allocates the limited number of communication resources among the plurality of communication units. The communication resources may comprise a TDM bus, a carrier frequency, a pair of carrier frequencies, or any RF transmission means. Of the communication resources, one is selected as the control channel which transceives operational data between the communication resource allocator and the communication units. The communication units, which may be portable radios and/or mobile radios, are arranged into talk groups, also called user groups, by commonality of use. For example, a user group may comprise communication units that are operated by a police department while another user group comprises communication units operated by a fire department.
In simulcast, multi-site trunked systems every call targets every site, and a communication resource is assigned to the call at each site. In such a system, all members of a user group have equal priority because each member is included in every conversation from its user group. More advanced trunked systems are more selective. They employ a technique such as dynamic site assignment, where a resource is assigned only to those sites that contain members of the user group associated with a call.
Other distinctions can be made. An individual unit may be designated as a critical or non-critical user. When any user, critical or non-critical, requests a resource, and a resource is available, the resource is assigned. When a resource is not available, however, a critical user must either wait for a resource or deal with a busy condition. If the resources are busy with non-critical users, the critical user suffers, as may others, especially if this is a life-threatening situation. A better balance of the needs of both critical and non-critical users is thus required.
Therefore, a method of efficient allocation of resources between critical and non-critical users is desired.